Estrogen receptor (ER) mediates estrogen signaling to promote breast cancer growth and thereby serves as a target for hormone therapy in ER+ breast cancer. DNA topoisomerase II1 (topo II1), on the other hand, maintains proper DNA topology and acts as a therapeutic target for both ER+ and ER- breast cancer. Although levels of intrinsic topo II1 protein correlate with sensitivity to its inhibitors, this protein is frequently depleted by its toxins and information about how topo II1 is maintained during the therapeutic stress is missing. p38 MAPKs consist of 1, 2, 3, and 4 proteins that are major kinase cascades to determine cellular response to upstream signals through regulating gene expressions. We showed during the past funding period that ER inhibits stress-induced cell death via c-Jun binding and our recent work further showed that ER specifically suppresses topo II drug-induced growth inhibition by attenuating p38 stimulations of topo II1 expression. It is proposed here that p38 MAPK activations maintain topo II1 expression that is antagonized by ER signaling, leading to increased topo II-drug toxicity in ER- breast cancer. This hypothesis is based on our preliminary studies showing 1) topo II inhibitors, but not taxol, are more potent in inhibiting ER- breast cancer growth that couples with increased intrinsic p383 but not topo II1 protein expression;2) there are also increased p381 phosphorylations linked to resistance of topo II1 proteins to topo drug-induced depletion in ER- over ER+ cells, indicating a topo II1 maintaining activity of both p381 and p383 that is antagonized by ER signaling;3) p383 antagonizes ER through binding / phosphorylating ER at S118;4) p381 and p383 trans- activate topo II1 in ER- but not ER+ breast cancer but 5) only p381 binds to topo II1 protein. These results together suggest that p38 MAPKs maintain topo II1 expression that is suppressed by ER signaling leading to a sustained topo II1 protein expression and increased topo II drug toxicity in ER- breast cancer. Experiments in this second funding period will demonstrate if 1) p381 and p383 cooperate to maintain topo II1 protein expression through their over-expression in ER+ and depletion in ER- breast cancer, 2) ER antagonizes p381/p383 stimulations of topo II1 expression by directly binding p383 proteins and indirectly attenuating p381 phosphorylation;and 3) p38 MAPK activities determine the growth-inhibitory efficacy of topo II drugs against human breast cancer in vitro and in mice. Topo II inhibitor-containing chemotherapy is known for its higher therapeutic activity against ER- breast cancer for several decades and mechanism involved for this selectivity however remains unknown. This proposal will test the hypothesis that p38 MAPK activation in breast cancer acts to maintain topo II1 protein expression during therapeutic stress that is antagonized by ER signaling leading to increased topo II drug activity in ER- breast cancer. These studies will reveal a novel function of p38 stress MAPKs in regulating topo II1 expression through cross-talking with ER signaling and results obtained will contribute significantly to breast cancer therapy. PUBLIC HEALTH RELEVANCE: Project Narrative Title: Estrogen receptor, p38 MAPKs and topo II1 in breast cancer PI: Guan Chen, MD, PhD Females are an important component of the armed forces / veterans who has made a great sacrifice for our country. Currently, women comprise more than 15% of the active duty military and the number of women veterans enrolled in the VA heath care system jumped from 226,000 in FY 2000 to 420,000 in FY 2002, an almost two-fold increase in a two-year period. VA health care system and medical researchers are obligated to provide a better service for them. Breast cancer is a leading female malignancy and it is estimated that one in seven women (about 237,500 female veterans) will develop breast cancer in lifetime. Breast cancer can be generally classified into two major classes, one expressing estrogen receptor (ER) called ER positive (ER+) and another without ER expression called ER negative (ER-). Although patients with ER+ breast cancer (about 70% of all patients) benefit from hormonal therapy such as tamoxifen, there have been no valid therapeutic targets for ER- breast cancers that account for about 30% of patients. Therefore, there is a great need to identify novel unique targets for ER- breast cancer for therapeutic intervention. It has been known for several decades that topo II drug-containing chemotherapy can achieve a better therapeutic response in ER- patients than ER+ counterparts. Since this protocol frequently consists of multiple agents, it is not clear which of these drugs are responsible for the high activity and whether there is a cellular target for this action. Our published work supported by the previous Merit Review showed that estrogen receptor (ER) expressed in breast cancer inhibits c-Jun-dependent stress-induced cell death (or growth inhibition) and our recent work using chemotherapeutic agents further showed that this occurs specifically to topo II inhibitors but not to taxol. These results suggest a possibility that it is a distinct topo II activity in ER+ and ER- breast cancer that results in a different therapeutic response to this therapy. This proposal is a continuation of our current studies and will focus on investigating how topo II1 activities are differently regulated and maintained in ER+ and ER- breast cancer in response to topo II drugs and thereby reveal novel mechanisms for the selective topo II drug toxicity against ER- breast cancer. Our preliminary results showed that the increased growth inhibition by topo II drugs in ER- breast cancer is surprisingly not due to an increased topo II1 protein expression that has been observed in many other types of cancer. Instead, we found that this is associated with an increased expression of p38 family protein p383 and sustained p381 phosphorylations in ER- breast cancer cells, indicating that p38 family proteins may cooperate to maintain the topo II1 expression and activity. This speculation is supported by our recent findings that p38 activations alone increase topo II1 expression in ER- but not ER+ cells. We will investigate in next funding- period a determinant role of p38MAPK activity in maintaining topo II1 expression and topo drug sensitivity in cell culture and mouse breast cancer model, and explore mechanisms by which ER blocks p38 MAPK stimulation of topo II1 expression. Topo II inhibitor-containing chemotherapy is known for its better activity against ER- breast cancer patients for several decades but mechanism for this selectivity however remains unknown. This proposal is to test the novel hypothesis that p38 MAPK activation acts to maintain topo II protein expression leading a sustained therapeutic response that is antagonized by ER signaling. Results obtained will uncover novel mechanism for the increased therapeutic activity of topo II drug-containing therapy against ER- breast cancer and will contribute significantly to new therapeutic developments through p38 MAPKs regulating topo II expression. Since breast cancer is a major malignant disease for female veterans, this study will contribute significantly to veteran's health care.